Input and Command-Line Arguments

Command-line arguments

Arguments after the file name arrive in *command-line-args*, a sequence of strings (or nil when there are none):

;; illustrative — run as: clj -M greet.clj Ada 3
(let [[name times] *command-line-args*
      name  (or name "world")
      times (if times (parse-long times) 1)]
  (dotimes [_ times]
    (println (str "Hello, " name "!"))))

Hello, Ada!
Hello, Ada!
Hello, Ada!

parse-long (Clojure 1.11+) turns a string into a number, returning nil if it isn't one. parse-double and parse-boolean work the same way.

Reading from the keyboard

;; illustrative — needs keyboard input
(print "What is your name? ")
(flush)
(def user-name (read-line))

(print "How old are you? ")
(flush)
(def age (parse-long (or (read-line) "")))

(if age
  (println (str "Hi " user-name ", next year you'll be " (inc age) "."))
  (println "That isn't a number."))

What is your name? Ada
How old are you? 36
Hi Ada, next year you'll be 37.

• read-line returns the next line, or nil at the end of input.
• print doesn't flush the output, so call (flush) before waiting for input, or the prompt may not appear.

Reading piped input

;; illustrative — run as: cat data.txt | clj -M count.clj
(let [lines (line-seq (java.io.BufferedReader. *in*))]
  (println (count lines) "lines"))

Environment variables and exit codes

(println (some? (or (System/getenv "HOME") (System/getenv "USERPROFILE"))))
(println "mode:" (or (System/getenv "APP_MODE") "development"))
(println (parse-long "42") (parse-long "4x2") (parse-double "2.5"))

true
mode: development
42 nil 2.5

(System/exit 1) ends the program with an error code that shells and CI systems can detect. Level 3 shows clojure.tools.cli for options like --verbose.